When optically scanning an object, for example, a printed text or image, in order to capture information stored therein, the object is generally exposed in a linear manner. The light radiated by the exposed line of the object has an intensity corresponding to the printed text or image. It is imaged by an imaging optical system on a receiver device—generally in linear or matrix form—by which the intensity fluctuations of the light are converted into electrical signals for further processing and display of the scanning result.
Examples of appliances based on this technology are flatbed scanners and read-out devices for X-ray films. In LED printers, LEDs (light-emitting diodes) arranged in the form of rows or matrices are activated selectively and the light radiated thereby is focused by an imaging optical system on a photosensitive medium from which it is then transferred to paper moving past the photosensitive medium.
In the above-mentioned applications it is the function of the imaging optical system optimally to image a long object, from which light is radiated, on a surface which has substantially the same geometrical dimensions as the object to be imaged.
Such an imaging optical system having a lens array is known, for example, from U.S. Pat. No. 6,088,164. In this imaging optical system light rays from a light source consisting of a plurality of linearly arranged light-emitting elements are focused on a surface of a storage medium by a lens array including a plurality of collecting lenses. The collecting lenses are arranged in the scanning direction such that two lines staggered with respect to one another are produced.
The disadvantage in this prior art is that because of the geometrical boundary conditions for the lens array (maximum constructional size) the numerical aperture is limited, so that the light-collecting power of the imaging optical system is low. This applies all the more if the lens array is constructed as a fibre array. In addition, crosstalk phenomena occur in this case.
A device for reading out information stored in a storage layer is known from DE 199 62 775. In this device the emission radiation radiated by an exposed X-ray cassette is imaged by optical imaging means on receiving means the numerical aperture of which, in relation to the stimulated line of the X-ray cassette, is greater in a direction transverse to the line than in the direction of the line. The imaging means consist of two cylinder lenses disposed parallel to the line stimulated and a lens array arranged between the cylinder lenses and having cylinder lenses perpendicular to the line stimulated.
This construction with cylinder lenses is very complex and therefore expensive because of the many aspherically ground surfaces.